High frequency system



March 2, 1943. w. H. UNGER 2,312,374 l HIGH FREQUENCY SYSTEM Filed Aug.l5, 1941 ATTORNEW Patented Mar. 2, i943 HIGH FREQUENCY SYSTEM William H.Unger, New York, N. Y., assignor, by

mesne assignments, to Patents Research Corporation, New York, N. Y., acorporation of New York Appiication August 15, 1941, Serial No. 406,961

(Cl. Z50-20) 6 Claims.

The present invention relates to high frequency systems embodyingfrequency responsive and/or control devices such as for improving thefrcquency stability, for detecting frequency modulated signals and forother uses and purposes.

One object of the invention is to provide an improved frequencyvariation response circuit or frequency discriminator for convertingfrequency changes of a signal current or potential into correspondingamplitude changes substantially free from response to spurious amplitudemodulation of the signal caused by noise or other interference, thusmaking it possible to dispense with a special limiter in a frequencymodulation receiver.

Another object is the provision of a simplified and highly efficientfrequency or tuning control system for use in a transmitter or receiverto maintain the transmitting or receiving frequency at a substantiallyconstant Value.

A further object is to provide a highly efficient and sensitivereceiving circuit for frequency modulated radio signals requiring aminimum of parts and circuit elements and especially suited for use inportable equipment.

These and further objects of the invention will become more apparentfrom the following detailed description taken With reference to theaccompanying drawing forming part of this specification and, wherein:

Fig. 1 is a circuit diagram for a frequency modulation receiverembodying the principles of the invention and specially suited forportable sets for communication over shorter distances, though notlimited thereto, and Fig. 2 is a transmitter circuit comprising asimplified frequency stabilizing circuit embodying the improvements ofthe invention. Fig. 3 is a graph explanatory of the function of thefrequency discriminator and Fig. 4 shows an alternative way of derivingthe discriminating voltage.

Like reference numerals identify like parts in the different views ofthe drawing.

Referring more particularly to the receiver shown in Fig. 1, frequencymodulated radio waves intercepted by an antennae I serve to generate asignal voltage across a tunable input circuit comprised of an inductioncoil II shunted by a variable condenser I2 and coupled between theantennae and ground I3 in a manner Well understood. The signal voltagedeveloped by the circuit I I, I2 is impressed between the signal inputgrid I4 and the cathode of a multi-grid electronic mixer tube orfrequency converter I5 provided with an additional oscillation controlgrid I6 excited from a local oscillation generator I'i in any suitablemanner to produce signal variations of intermediate frequency in theplate circuit of the tube. The intermediate frequency signals areselectively amplified by means of an amplifier especially designed forthe intermediate frequency and comprising, in the example shown, twocascade connected pentode stages I8 and I9 coupled by means of band-passtype coupling networks Z and 2i' of known design.

The amplified intermediate frequency signals supplied by stage I9 areapplied to a frequency detector or discriminator 2@ of the general typedescribed in U. S. Patent 2,208,071 which serves to convert thefrequency variations of the received signal waves representing theintelligence or message being transmitted into corresponding currentvariations suitable for energizing an output device such as a loudspeaker or a pair of head phones 2I as shown in the drawing.

The receiver shown utilizes tubes provided with directly heated cathodessuitable for operation from a low voltage heating battery 22 markedA-land A-. The anodes and screen grids of the tubes are energized from ahigh voltage battery marked B+ and B- in a manner Well understood.Suitable anode, screen and cathode bypass condensers as Well as voltagedrop or bleeder resistors are provided in the anode and screen gridleads as shown to prevent self -oscillation and other undesirableeffects in accordance With the customary design practice of radioreceiving circuits.

The discriminator tube 20 comprises a cathode 24 followed in the ordernamed by a first control or signal input grid 25, a second control grid26 enclosed by a positively biased screen grid 21' and an anode or plate28. Intermediate frequency signal voltage supplied by the amplifierstage I 9 is impressed between the grid 25 and cathode 24 of thediscriminator tube by Way of a band-pass coupling network comprisingprimary and secondary tuned circuits 29 and Sii, respectively. In thismanner the electron space current passing from the cathode 24 to theplate 28 is subjected to initial variations in accordance with the highfrequency signals being received. The control grid 26 is connected tothe cathode by way of a resonant impedance such as a parallel tunedcircuit 3| resonant to the intermediate frequency of the receiver.

As described in the above-mentioned patent, a displacement current willbe induced in the external circuit of the grid 26 by the action of avirtual cathode or concentrated electron space it will oiier a highnon-reactive impedance to the displacement current resulting in apotential upon the grid 26 substantially in phase with the displacementcurrent which, in turn, is 90 out of phase with the space chargeiiuctuations or the potential upon input grid 25. If the latter isfrequency modulated the circuit 5i will oier increasingly capacitativeor inductive reactance to the displacement current in proportion to andaccording to whether the signal frequency deviates in one or the otherdirection, respectively, from the carrier or center frequency inaccordance with the variation of a modulating signal. Accordingly,therefore, the phase of the voltage developed upon grid 2t will beincreased beyond or decreased below the normal 90 phase relation to theinput signal upon the grid 25.

As a result of the above operation the electron space current passingfrom the cathode to the plate 28 will be subjected to a double controlby indentical alternating potentials upon grids 25 and 26 but having arelative phase varying both in sign and magnitude in accordance with thefrequency departure of the impressed signals from the carrier or centerfrequency. A double control of this type will cause a component to beproduced in the output or plate cur-rent proportional to the product ofthe control potentials upon grids 25 and 25 which component, in turn,will include a direct current term varying substantially in proportionto the relative phase be-r tween the control potentials, i. e., inproportionto the frequency changes to be detected `or converted. In Fig.3 the average plate current ip is shown plotted as a function of thefrequency f. As is seen, the current ip varies in either direction withrespect to the normal (quiescent) plate current io corresponding to thelcarrier or center frequency fo to which the circuit 3l `is resonantsubstantially linearly within a range b between the lower and upperlimit frequencies fl and f2, respectively. The steepness of the curve,vthat is, the sensitivity of the discriminator may be increased byincreasing the Q or decreasing the damping of the circuit 3l. `In thelatter case the range b will be decreased and in practice the circuit 3|is so designed as to provide optimum sensitivity for a given maximumfrequency deviation Without exceeding the straight line portion of thecurve and introducing distortion. The average screen current z'sgundergoes a variation similar to the plate current but in an oppositedirection as shown by the dotted curve. Thus, if the input signals arefrequency modulated in accordance with variations of sound waves suchVas speech, a demodulated current will appear in the -plate circuitsuitable for operating the headphones 2 I.

There are principally two methods fof operat ing a space charge typediscriminator described hereinabove. According to one method, the-signalinput grid 25 is so biased and operated that substantially -no gridcurrent is drawn or, in

other words, that the grid acts as a pure control electrode (class Aoperation). In this case the total emission current from the cathode 24after being subjected to the control by the grid 25 will pass towardsthe screen 2'! to form a virtual cathode between the latter and thecontrol grid 25. Since both control grids are operated on the straightline portion of their operating characteristics l(grid voltage vs. platecurrent) substantially no rectication Will take place and the tube willbe substantially non-responsive to spurious amplitude modulation of theimpressed input signals.

According to another method, the input grid 25 vi's so biased andoperated and the amplitudes of the input signal so chosen that asubstantial grid current is drawn during the positive half- `cycles ofthe input signal (class C operation), whereby grid 25 acts as an outputelectrode and the space current passing to the screen 21 will be equalto the difference between the total emission from the cathode and thecurrent flowing through the grid circuit in such Va :manner that againthe current passing through the screen 2i will be subjected tovariations iny -accordance with the impressed input frequency. This modeof operation has been yfound 'to be superior and more efficient than theone above mentioned utilizing grid 25 as a pure `control element,provided a signal of suiiicient amplitude is applied to the input grid25. In the latter case, the 'output current variations 'obtained fromthe discriminator with the proper adjust'- rnent of the circuit elementsand operating -conditions will be sufficient to directly energize 'aundesirable amplitude 'changes of an impressed frequency modulatedpotential caused by iiuctuation or impulse noise or other interferencewill be detected and cause interference inthe head set or other outputdevice. To 'obviate this i defect in accordance withthe presentinvention,

the direct current impedance of the grid circuit is kept at a minimum byavoiding anygrid bias resistance and by making the oh'mic vresistance ofthe secondary 30 of the input Vtransformer as low as possible to preventiany rectified current from producing an appreciable 4voltage drop inthe grid circuit. For this reason the .grid is biased by the aid `of a'C-battery 32 icy-'passed by 'a condenser 53 which also serves to biasthe second control grid 26 to a potential negative with respect to thecathode. The same `consid'- eration's apply to the control grid 26,which'inay be either biased so as to draw no electron current or which,if current is drawn, is returned to the cathode through the smallestpossible D. C. impedance to prevent any rectified current from buildingup any appreciable Voltage drop.

If desirable, separate biasingsources 'may be provided for the gridsY 25and 25. Condenser 3l! serves to by-passthe high frequency componentsscribed, itis also possible to ldevelop a radio fre# quency Voltage bythe insertion of a high frequency impedance such as a tuned circuit inthe external plate circuit. This radio frequency component represents anamplitude modulated signal, that is, the tube acts as a converter'offrequency modulation to amplitude modulation. The latter may be detectedin any suitable manner such as by the aid of an additional diodeembodied in the tube to obtain an output varying substantially accordingto the curve of Fig. 3 as described in U. S. Patent 2,248,197, issuedJuly, 1941, and entitled Frequency variation response circui to whichreference is made for further detail.

A receiver of the type shown in Fig. 1 utilizing the 1R5 type midgettube available on the market as oscillator, mixer and discriminator andtwo 1T4 types as I. F. amplifiers was found to operate satisfactorily.With a signal strength enough to provide 50 micro-amperes of gridcurrent with 3 volts of bias, the plate current was found to undergo atotal variation of nearly .5 milliampere out of an average current ofabout .'7 milliampere. If a load resistance is used in the plate circuitthen the average current and the resulting variations are reduced. Thus,in case of an ohmic load resistance of 50,000 ohms a 300 microvoltsignal applied to the rst grid of the mixer tube with a total frequencydeviation of 3 k. c. was found to provide a five volt peak-to-peak audiosignal in the plate circuit of the discriminator. As pointed out, it isimportant to provide a low resistance return path for the grids of thediscriminator to substantially prevent the tube to respond to variationsin amplitude as well as in frequency. In this manner a special amplitudelimiter preceding the discriminator as used in the standard frequencymodulation receivers may be dispensed with, resulting in substantialsaving of parts and simplification of the receiver, which is of specialimportance for portable sets wherein low weight and bulk are to be givenprime consideration.

As pointed out above, the tuned circuit 3|, which may be replaced by anyother suitable resonant impedance device, should have the highestpossible impedance at the resonant, that is, the I. F. frequency to getthe greatest sensitivity compatible with the maximum frequency deviationof the input signals.

In a system of the type described, it is of paramount importance thatthe center or carrier frequency of the input signal voltage applied tothe discriminator should always correspond to the center tuningfrequency of the I. F. amplier and the discriminating circuit 3l. If thecarrier or center frequency deviates from its assigned or xed value, dueto oscillator drift or other causes, the operating point O, Fig. 3, ofthe discriminator will be displaced, resulting in considerable outputsignal distortion in addition to other distortions produced in the I. F.amplifier as is well understood. In order to avoid this drawback thereis provided in Fig. 1 a most simple and efficient frequency controlarrangement to maintain the carrier or center frequency at all timesexactly in line with lthe center resonant or tuning frequency of thecircuit 3 l, that is the I. F. frequency of the receiver.

For this purpose, there is inserted in the plate circuit of thediscriminator a resistance 3B bypassed to ground through an audiofrequency condenser 35 to develop a frequency control (AFC) voltage dueto slowA carrier. or center frequency variations at a rate belowy thelowest audio frequency being transmitted due to oscillator drift andother causes. This AFC voltage is utilized to directly readjust thefrequency of the local oscillator I7 to correct for any initial cen; teror carrier frequency deviation of the signal potential impressed uponthe discriminator input from the fixed or normal value. For this purposethe automatic frequency control (AFC) voltage developed by theresistance 36 in the plate circuit of the discriminator is directlyutilized to energize the plate of the triode oscillator tube 3l todetermine or control the oscillating frequency.

The oscillator shown is of the standard regenerative type comprising afrequency determining grid tank circuit comprised of a variablecondenser 38 and an induction coil 39, a feedback or tickler coil 40 inthe plate circuit and grid bias resistance 4I. Attempts have alreadybeen made to apply the discriminating control voltage directly to theoscillator grid for the control of the oscillator frequency and acareful choice of circuit constants has enabled the control of thefrequency to be obtained over a restricted range. There are, however,many objections `to this method. A more favorable method of varying theoscillating frequency is to control the plate or -screen voltage of theoscillator. This, however, in the past required a separate tube sincethe discriminating (AFC) voltage produced by the known discriminatorswas insufcient to energize either the plate or screen of an oscillatingtube.

By the combination of the present invention all the previousdiiiiculties are avoided, while greatly simplifying the circuitarrangement by the elimination of an additional control or reactancetube. The discriminator used by the invention supplies a sufficientlyhigh control voltage which is di.- rectly impressed as operating voltageupon the plate of the oscillator for the control of the oscillatingfrequency. As is well known, the change of the oscillating frequency isdue to a reflected reactance fed back from the plate into the gridcircuit and varying in proportion to the conductance of the tube whichaccording to the invention is controlled by varying the plate supplyvoltage. A similar effect may be obtained by varying the screen voltagewhen using a tetrode for the local oscillator.

In most cases the reflected reactance in the oscillator is of acapacitative nature due to the inherent grid-plate capacity and variesdirectly in proportion to the trans-conductance of the tube, that is, inthe present case the plate operating voltage. Thus, if the oscillatorfrequency is greater than the signal frequencyv and supposing that theoscillator frequency increases due to drift, temperature, humidity andother influences, then the intermediate frequency will also increase,resulting in a decrease of the average discriminator plate current ip,as seen from Fig. 3. Consequently, the oscillator plate voltage willrise resulting in an increased reflected reactance in the tank circuit38, 39, thus, in turn, causing a decrease of the oscillator frequencycounteracting the initial frequency increase. On the other hand, if theoscillator frequency is below the signal frequency the correction willbe in the opposite sense and the oscillator drift increased, thusrendering the system unstable. In the latter case, automatic tuningcorrection may be obtained by deriving the control (AFC) potential fromthe screen grid rather than the plate of the discriminator, as shown inFig. 4. Alternatively, when using an oscillator wherein the-reflectedreactance varies inversely with the trans-conductance,automatic'frequency control maybe obtained with the oscillatingfrequency being lower than the signal frequency by Vusing .the platevoltage of the discriminator in the manner shown inFig. 1.

In order to prevent resi-dual audio frequency variations from beingimpressed upon the oscillator plate it is important to include asuitable audio ilter in the AFC circuit, such as in the form lof aseries choke coil 42 and bypass condenser 43, as shown, otherwise theaudio output will be greatly attenuated. This type filter circuit shouldlbe designed with regard to the phase shift encountered since if thephase shift is more than 90 while the attenuation is not complete, 'theset will oscillate at an audio frequency. A choke coil is used since theoscillator plate current in the above example is approximately 2 maand aresistance of higher value to be effective would reduce the oscillatorplate voltage too much. In an arrangement of this type it was found thatany changes in the antennae would not appreciably affect the frequencyof the oscillator. The set was found to be quite vstable and the AFCeffective even fon fairly weak signals.

It will be evident that the frequency discrim inator and automatictuning or frequency control `arrangement are not limited to a receiverof the specific .type described for illustration but Will have numerousother uses and applications in combination with o-ther circuits, bothfor transmitting and receiving. Thus, the discriminator vmay be employedwithout the automatic tuning feature or the automatic frequency controlsystem described may be incorporated in an amplitude modulation receiverwithout dep-arting from the spirit of the invention. Moreover, thefrequency Vcontrol may serve for stabilizing the center `rlcarrierfrequency of an amplitude or frequency modulated transmitter.

An arrangement of the latter type is shown in Fig. V2, wherein, item 50represents a master oscillator tube provided with an oscillating tankcircuit 5I connected tothe tube to form a well known regenerativeoscillator system. A quadrature control potential derived from the tankcircuit 5| by Way of a phase shift arrangement comprising a resistance52 and condenser .in series is applied to the control grid 56 of asocalled reactance control tube 55 by way `of a coupling condenser 54.As is well known, the plate cathode path of `such a tube simulates areactance which is 'connected across the tank circuit 5I by way `of afurther coupling condenser 58. The reactance of the tube 55 iscontrolled in Yany suitable manner in accordance with the modulating:signal currents impressed by way of Aa low frequency transformer upon asuitableV `control element, such as the suppressor grid as shown. Thefrequency modulated oscillations are passed :in amanner known Vthrough afrequency multiplier 59 to a power amplifier 66 feeding an Aantenna 62by Way of coupling transformer BI.

A portion ofthe amplified energy is impressed upon the ihputgrid of:discriminato-r S4 through a resonant coupling transformer 63. Thediscriminatorisxof substantially the saine type `and operated in Vamanner similar to that described in connection with Fig. 1. Item 65represents the vdiscriminating circuit which, in this case, is resonantto the centeror carrier frequency being transmitted. Since thediscriminator is not used as a demodulator, the Q of the circuit 65maybe chosen sufficiently Vhigh to obtain an extreme frequencysensitivity. Alternatively, the circuit 65 may be replaced by a piezoelectric crystal to further increase the sensitivity of the tube tofrequency changes. Item 66 is the output resistor corresponding toresistor 3B in Fig. 1 which acts a variableV bleeder or drop resistancein the 'anode supply lead for the oscillator tube 50. lIhe frequencystabilization functions in substantially the same manner as in the caseof Fig. 1 by controlling the anode potential of the oscillator in amanner to counteract initial deviation of the transmitting frequencyfrom the assigned carrier or center frequency to which the circuit 64 isresonant. This system has the f-urther advantage over known arrangementsin that the frequency modulation and stabilization are separated,resulting in a further increase of the operating stability of thetransmitter. Depending upon the particular type of `oscillator used, theplate (AFC) voltage is derived from either the plate oir the screen ofthe dis-criminator to ob-tain a frequency correction in the propersense. In Fig. 4 there is shown a discriminator wherein the screen gridcircuit includes a resistance 68 by-passed to cathode by condensers 69and 10 and serving to develop a varying (AFC) voltage for energizing theoscillator in Fig. l vor 3. The ,plate of the discriminator in this caseis connected to a source of high potential either directly or through aresistance 'il suitably icy-passed for high frequency.

According to a modification, the connection of the grids 25 and 26 ofthe discriminator may ybe interchanged, that is, the input signal may beapplied to grid 25 and the discriminating circuit connected to grid 25without substantially affecting the operation of the discriminator,except for a reversal of the polarity of the curves shown inV Fig. 3.The filter in .the circuit in Fig. 2 may be so designed as not only topass slow variations due to oscillator frequency drift, but to Vpassmodulating Vfrequency currents as well to apply a. negative feedbackvoltage to the oscillator plate to improve the operation of thetransmitter bly diminishing noise and distortion in a manner Wellunderstood by those skilled in the art. In this case the circuit 55should be designed With a Q to provide an operating range b to cover themaximum frequency deviation caused by the modulating signals applied tothe terminals a-b.

In Fig. rl the grids of the mixer I5 and the oscillator are directlyconnected but they may vbe coupled in any suitable manner, such ascapacitatively, in which case a grid leak should be provided for thegrid i6 'of the mixer. The variable condenser i2 of' the .input circuitand the condenser 38 of the oscillator tank circuit are arranged lforuni-control, as indicated by the dot and dash lines .and the circuitssuitably tracked to .produce the same intermediate frequency for alltuning adjustments nin a manner well understood by those skilled i-n theart.

It will be evident from the foregoing that fthe invention is not limitedto the specific circuit arrangements and details shown and disclosedtherein for illustration, but that the underlying novel principles willbe susceptible of numerous variations and modifications coming within abroader Vscope and spirit of the invention as delined Vin `the appendedclaims. The specification and drawing are accordingly to be regarded inan illustrative rather than a limiting sense.

I claim:

l. A frequency variation response circuit comprising an electrondischarge tube provided with a cathode, an anode and a plurality ofgrids, an input circuit connected to one of said grids and said cathode,an output circuit connected to said anode and said cathode. meansincluding resonant impedance means connected between said cathode andanother of said grids for causing variations of the anode currentsubstantially in proportion to the relative frequency departure of animpressed input signal frequency from the resonant frequency of saidimpedance means, said input circuit being designed to offer a low enoughimpedance to the modulation frequencies of an undesired amplitudemodulation of said input frequency to render the effect thereof on theanode current due to rectification in said input circuit substantiallynegligible.

2. A frequency variation response circuit comprising an electrondischarge tube provided with a cathode, a rst control grid, a positivelybiased screen grid, a second control grid and an anode, all arrangedsubstantially in the order named, an input circuit connected to one ofsaid control grids and said cathode, means whereby electron current isdrawn by said last control grid during the positive signal half cycles,an output circuit connected to said .anode and said cathode, resonantimpedance means connected to said cathode and the other of said controlgrids for causing variations of the anode current substantially inproportion to the relative frequency departure of an impressed inputsignal frequency from the resonant frequency of said impedance means,said input circuit being designed to offer a low enough impedance to themodulation frequencies of an undesired amplitude modulation of saidinput frequency to render the effect thereof on the anode current due torectification in said input circuit substantially negligible.

3. A frequency variation response circuit comprising an electrondischarge tube provided with a cathode, a first control grid, apositively biased screen grid, a second control grid and an anode, allarranged substantially in the order named, an input circuit connected tosaid first control grid and said cathode, an output circuit connected tosaid anode and said cathode, resonant impedance means connected to saidsecond control grid and said cathode, whereby said second control gridis excited by electron coupling from an impressed input signal frequencyat varying relative phase thereto in dependence upon the relativefrequency departure of said input frequency from the resonant frequencyof said impedance means to cause anode current variations in accordancewith said frequency departure, said input circuit being designed tooffer a low enough impedance to the modulation frequencies of anundesired amplitude modulation of said input frequency to render theeffect thereof on the anode current due to rectincation in said inputcircuit substan-A tially negligible.

4. A frequency variation response circuit comprising an electrondischarge tube provided with a cathode and anode and a plurality ofgrids, an input circuit connected to one of said grids and said cathode,means whereby electron current is drawn by said last-mentioned gridduring the positive half cycles of an impressed input signal frequency,an output circuit connected to said anode and cathode, means includingresonant impedance means connected between said cathode and another ofsaid grids for causing variations of the anode current substantially inproportion to the departure of said input frequency from the resonantfrequency of said impedance means, said input circuit being designed tooifer a low enough impedance t0 the modulation frequencies of .anundesired amplitude modulation of said input frequency to render theeffect thereof on the anode current due to recticatio-n in said inputcircuit substantially negligible.

5. In a frequency discriminator, resonant impedance means having aresonating frequency relatively variable with respect to a highfrequency signal potential, an electron discharge tube comprising meansfor producing an electron space current, means including a iirst controlelectrode for said tube and an input circuit connected thereto forvarying said space current in accordance with said signal potential, asecond electrostatic control electrode in said tube, means forconnecting said impedance means to said second control electrode,further means for producing a concentrated electron space chargeadjacent to said second control electrode to excite the same throughelectron coupling with said space charge by said signal potential atvarying phase thereto in accordance with the relative frequencydeparture of said signal potential from the resonating frequency of saidimpedance means, an output circuit for said tube, and means operativelyassociated with said output circuit for developing energy varying inproportion to said frequency departure, said input circuit beingdesigned to offer low impedance to the modulation frequencies of anundesired amplitude modulation of said signal potential to render theeffect thereof on said output current due to rectication in said inputcircuit substantially negligible.

6. In a frequency discriminator, resonant impedance means having a iixedresonating frequency, an electron discharge tube comprising means forproducing an electron space current, means including a rst controlelectrode for said tube for varying said space current `in accordancewith a high frequency signal potential having a frequency varyingrelative to said fixed frequency, a second electrostatic controlelectrode in said tube, means for connecting said impedance means tosaid second control electrode, further means for producing aconcentrated electron space charge adjacent to said second controlelectrode to excite the same through electron coupling with said spacecharge by said signal potential at varying relative phase thereto inaccordance with the frequency departure of said signal potential fromsaid fixed frequency, an output circuit for said tube, and meansoperatively associated with said output circuit for developing energyvarying in proportion to said frequenci7 departure, said input circuitbeing designed to offer low impedance to the modulation frequencies ofan undesired amplitude modulation of said signal potential to render theeffect thereof on said output circuit due to rectification in said inputcircuit substantially negligible.

WILLIAM I-I. UNGER.

